Sonoran Desert Toad, Incilius alvarius
Sonoran Desert Toad, Incilius alvarius. Toad photographed within the confines of the Arizona-Sonora Desert Museum, Tucson, Arizona, February, 2022.
The Sonoran Desert Toad, Incilius alvarius, is a member of the Bufonidae Family of toads. The Bufonidae Family consists of six hundred eleven members placed into fifty-two genera. There are thirty-nine species in the Incilius genus. They are also known as the Colorado Desert Toad. In Mexico, they are referred to as Sapo del desierto de Sonora and Sapo del Río Colorado.
Sonoran Desert Toads are one of the largest toads native to North America. Their stout bodies are uniformly green to greenish-gray dorsally, with a creamy white ventral surface and neck. Large white tubercles, or “warts”, are present behind the angle of their jaw. Aside from large parotoid glands and a few large lumps on their hind legs, they have relatively smooth skin. Above each of their eyes curves a distinct cranial crest. The length of each parotid gland is twice the parotid width, and equal to the distance from the nostril to the tympanum. Sexual dimorphism is displayed during the breeding season when males develop darkened, thick calluses on the inside of their thumbs and forelimbs. Adults reach between 10 cm (3.9 inches) and 20 cm (7.9 inches) in snout to vent length, and they weigh up to 300 g (10.6 oz).
Sonoran Desert Toads are nocturnal and active from late May to September, primarily during the summer rainy season. During the daylight and periods of intense heat, they take refuge in rodent burrows and rocky outcrops. How this species aestivates or survives over long periods of heat and cold is not known. However, observations suggest they may enter a state of torpor. Breeding occurs from late spring through early fall with the onset of the summer rains. While breeding activity is stimulated by rainfall, it is not dependent upon it. Reproduction is aquatic and occurs in seasonal and permanent pools, stock tanks, and irrigation ditches. The reproductive cycle of Sonoran Desert Toads is similar to that of most North American frogs and toads. Mature adults come into breeding condition and migrate to ditches and rain pools where males call to advertise their fitness to competing males and prospective females. These calls have been described as sounding somewhat like a ferryboat whistle. Individuals may travel up to several hundred meters to find a suitable water source. In addition to producing advertisement calls individually and as a chorus, males will travel in search of females or sit without calling and wait for females to come close enough to grab them. Males and females pair up in axillary amplexus in the water, with the male grabbing the female around her shoulders or armpits. The female lays her eggs as the male fertilizes them externally. An average of 7,500 to 8,000 eggs are laid in long, single, jelly-coated strings or tubes. No additional parental care is provided to the offspring. The eggs hatch into gray to golden brown tadpoles, which feed on algae and detritus in the water and grow up to 5.7 cm (2.2 inches) in length. Metamorphosis takes place quickly, usually in less than a month, and the tadpoles grow four legs, lose their tails, and emerge onto land where they disperse into the surrounding territory. Recently metamorphosed toadlets are tan to green in color with orange or red spots on their dorsum. Adults live an average of four to five years in the wild, and over ten years in captivity.
Similar to most toads, Sonoran Desert Toads consume anything that they can overtake and capture. Their diet primarily consists of insects, spiders, scorpions, centipedes, snails, beetles, ants, and termites. They are also known to consume small lizards, other amphibians, and mice. Sonoran Desert Toads are capable of moving very quickly with huge leaps and by running on all four legs. They are preyed upon by large reptiles, mammals, and birds. In their defense, Sonoran Desert Toads possess extremely potent defensive toxins that are released from several glands (primarily the parotids) in their skin. They inflate to increase their body size and assume a butting pose, aiming their parotid glands at the adversary. Animals that harass this species are generally intoxicated through the mouth, nose, or eyes. These toxins have hallucinogenic properties and may cause paralysis or even death.
Sonoran Desert Toads occur from extreme southeastern California, southern Arizona, and extreme southern New Mexico in the United States, south into northwestern Chihuahua, Sonora and northwestern Sinaloa in Mexico. This species has been nearly extirpated in California, and populations appear to have also declined in New Mexico. Their elevational range extends from sea level to up to 1,600 m (5,250 feet). Sonoran Desert Toads inhabit arid mesquite-creosote bush lowlands, arid grasslands, mountain canyons, and oak, sycamore, and walnut woodlands. While they are often found near permanent water, their location is not dependent upon it.
From a conservation perspective, the Sonoran Desert Toad is considered to be of Least Concern due to their wide distribution, presumed large population size, and tolerance to a degree of habitat modification. While populations may face localized challenges, no major threats have been identified for this species. If encountered, Extreme Caution should be used to avoid direct contact. Dog owners should be particularly cautious, as the toxins they possess are strong enough to kill full grown adult dogs. Symptoms of intoxication include excessive salivation, irregular heartbeat and gait, and pawing at the mouth. If a dog displays any of these symptoms, the use of a garden hose to rinse their mouth from back to front might be beneficial and and a veterinarian should be consulted. Since the toxins of the Sonoran Desert Toad are know to cause hallucinations in humans, they have been involved in reports of toad licking and the smoking of dried parotid gland secretions. As a result, some states classify this species a controlled substance and have passed laws against such actions.